In the 3GPP (3rd Generation Partnership Project) which is the international standardization body for the third generation mobile communication systems, standardization of E-UTRA (Evolved-UMTS Transmitter Radio Access) is fostered. The E-UTRA aims at speeding up the 3.5 generation, UTRA, and is positioned as the 3.9 generation called LTE (Long Term Evolution).
In the LTE, communication is carried out based on spatial multiplexing such as MIMO (Multi-Input Multi-Output) to achieve fast and mass information transfer and improve the frequency use efficiency. Further, the fourth generation, IMT-Advanced, which involves a greater number of spatial multiplexing channels and a wider band has been studied. The MIMO is the communication system that uses a plurality of antennas to increase paths based on spatial multiplexing and improve the throughput. This communication system also uses the same frequency to provide a satisfactory frequency use efficiency. MIMO whose number of inputs is a and whose number of outputs is b is called a×bMIMO. In the LTE, for example, the quantity of antennas at a base station (maximum spatial multiplexing channels) and the quantity of antennas of a terminal (maximum of four), which is 2×2MIMO (4×4MIMO at a maximum).
A terminal in the LTE acquires the quantity of antennas at a base station from information included in a PBCH (Physical Broadcast CHannel) which is a common control channel to globally inform control information unique to systems and cells. This terminal uses an RS (Reference Signal) to calculate a spatial matrix from the RS received at each antenna, uses a PUCCH (Physical Uplink Control CHannel) to inform the base station of a PMI (Precoding Matrix Indicator), RI (Rank Indicator), and CQI (Channel Quality Indicator). The base station decides the precoding and transmission mode based on the PMI, RI and CQI, and uses a PDCCH (Physical Downlink Control CHannel) to inform the terminal of the result of the decision.
When there are a large number of low-correlation spatial multiplexing channels available, for example, communication at a maximum of about 300 Mbps is possible in DL (Down Link). When the radio wave environment is poor, however, a base station carries out communication with transmission diversity in transmission mode. In the latter case, transmission of same data from two antenna ports may not improve the throughput, but can enhance the redundancy to achieve stable communication.
Unexamined Japanese Patent Application KOKAI Publication No. 2008-166855 discloses the configuration that carries out communication using a plurality of antennas.
With the speed of mobile communication improved, the use of spatial multiplexing typified by MIMO further improves the throughput and the frequency use efficiency. However, increasing the number of spatial multiplexing channels brings about various problems such that the quantity of antennas is increased, and dissipation power is increased by an increase in the amount of signal processing. Therefore, there is a limit to the quantity of antennas mountable to a portable terminal which is used in mobile communication.
It is therefore desirable to increase the quantity of antennas in carrying out communication. However, the technique described in Unexamined Japanese Patent Application KOKAI Publication No. 2008-166855 is about where antennas are mounted, such as an external antennaslement is provided at the battery pack of a cellular phone. It is therefore difficult to keep a satisfactory communication state by flexibly changing the antenna configuration according to various situations.